Spectrophotometers operating in various portions of the ultra-violet through near infrared wavelength range, and Fourier Transform Infrared spectrometers operating in the longer infrared wavelength range are well-known metrology tools useful in a variety of scientific or technological fields, for example, biology, geology, forensics, nutrition science, medicine, and semiconductor processing.
Spectrophotometers are instruments for measuring spectral transmittance or reflectance properties of a sample. Typically, spectrophotometers use a diffraction grating or prism to separate light into a spatially distributed spectrum. The spatially distributed spectrum of light is used to generate an intensity profile of a sample. Measurement and analysis of the intensity profile yields information about the sample or an overlying film including transmittance, reflectance, index of refraction, and thickness. The wavelengths of light typically used by a spectrophotometer falls in the ultra-violet (UV), about 0.2 micrometer (.mu.m), through the near-infrared (NIR), about 1.5 .mu.m, which encompasses the visible (VIS) range.
In the measurement of film-thickness using a UV-NIR spectrophotometer, the film must be at least
partially transparent to the particular wavelengths of light being used. Light is reflected from both the front and rear surfaces of the film. Depending on the wavelength of light, the film's index of refraction, and the thickness of the film, the reflected light rays will combine, constructively, destructively or somewhere in between, in the plane of the displayed or detected spectrum. Thus, a ripple or fringe pattern is generated. From the exact shape of the fringe pattern, the thickness of the film can be determined. With a thicker film, there is a greater number of fringes in the spectral pattern relative to a spectral pattern of a thinner film. However, because spectrophotometers have a spectral resolution limit, there is a film-thickness above which a spectrophotometer cannot effectively resolve the fringe pattern. Consequently, the UV-NIR spectrophotometer may be unsuitable to analyze a thick film. Further, some materials are opaque in all or some of the wavelengths in the UV to NIR range. Thus, the UV-NIR spectrophotometer is also unsuitable to measure these materials.
One solution to the problems posed by materials that are opaque in the UV to NIR range or too thick for a UV-NIR spectrophotometric measurement is to extend the wavelength range into the infrared (IR) region. Hence, a Fourier Transform Infrared spectrometer is often used for thick-film thickness measurements.
A Fourier Transform Infrared (FTIR) spectrometer uses a Michelson interferometer, which includes an IR source, a beam splitter, and two plane mirrors (one fixed and one moving) and after mathematical processing produces a spectrum of the light coming from the sample. A FTIR spectrometer is particularly useful at IR wavelengths from about 2 .mu.m to 1 mm. A FTIR spectrometer detects the absorption of the IR light that is either transmitted through or reflected by the sample. In the IR range, absorption of light is associated with molecular bonding and, thus, valuable compositional information can be obtained. Because the spectrum intensity variations can also be caused by interference effects of light reflecting from different interfaces, film-thickness information can also be extracted, including thicknesses much greater than may be measured with a UV-NIR spectrophotometer.
Commercially available FTIR spectrometers are typically self-contained, bench-top systems that require transferring a sample onto a dedicated inspection location, such as an enclosed sample chamber. Purging of the FTIR optical path with an inert gas is typically performed to eliminate atmospheric water-vapor and carbon dioxide that can cause major absorption peaks in FTIR spectra. Thus, FTIR spectrometers are often enclosed chambers. Examples of FTIR spectrometers are the Century Series FT-IR Spectrometers made by Bio-Rad located in Cambridge, Mass., the Epitaxial Layer Thickness Monitor MappIR by PIKE Technologies, located in Madison Wis., the MB Series of FTIR Spectrometers by Bomem, located in Quebec, Canada, the Genesis Series FTIR by Mattson, located in Madison, Wis., the M-Research Series and SPR Prospect IR spectrometer by Midac Corporation, located in Irvine Calif.
Obtaining results using a UV-NIR spectrophotometer and FTIR spectrometer requires having two separate, dedicated instruments with, of course, sufficient table or floor space to accommodate both. Separate UV-NIR spectrophotometer and FTIR spectrometer measurements of the same sample area are desirable, for example, where composite thick and thin films are present over the sample area or where information relating to different characteristics of the sample area are desired, such as film thickness and compositional information.
FIGS. 1A and 1B are schematic diagrams of a UV-NIR spectrophotometer 10 alongside a FTIR spectrometer 20. As shown in FIG. 1A, a sample 14 is positioned under the axis 12 of spectrophotometer 10. Sample 14 is on a stage 16 that is used to precisely position sample 14 under axis 12 as well as reposition sample 14 under axis 22 of FTIR spectrometer 20 (sample 14 is shown located under axis 22 by broken lines). FIG. 1B similarly shows sample 14 under axis 12. However, two separate stages 18, 28 are used to correctly position sample 14 under respective axes 12 and 22. Thus, a mechanism to transfer sample 14 from stage 18 to stage 28 is required. Thus, the use of spectrophotometer 10 alongside FTIR spectrometer 20 requires the use of a large stage 16 (as shown in FIG. 1A) or moving sample 14 from stage 18 to separate stage 28 (as shown in FIG. 1B). The task of moving sample 14 from one instrument to the other and finding and positioning the same measurement area of sample 14 under axes 12 and 22 is difficult and time-consuming, which consequently reduces throughput.
Further, the use of either one large stage 16 or multiple stages 18 and 28 requires a large amount of space. However, in semiconductor processing, the minimization of the footprint of a clean room tool is important because of the high fabrication and maintenance costs per square foot in a clean room. Consequently, the use of completely separate spectrophotometer and FTIR spectrometer instruments in a clean room is undesirable because of the space required. Using a large stage or multiple stages is undesirable because of higher material costs, as well as the time required to reposition the sample under each instrument. Moreover, the data correlation for the areas measured by the separate devices is limited by the accuracy with which the sample area can be repositioned in the subsequent metrology instrument.
Combining spectrophotometer 10 and FTIR spectrometer 20 is difficult because the units use different hardware, such as different light sources, i.e., UV-NIR sources versus an IR source, and different detectors, along with different techniques for analysis. Additionally, FTIR spectrometer 20 is typically contained within a sealed enclosure (not shown) to eliminate atmospheric interferences that may cause unreliable and inaccurate results. Moreover, spectrophotometer 10 and FTIR spectrometer 20 are typically used for separate types of measurements, e.g., among other characteristics of the sample spectrophotometer 10 measures thin film thicknesses, while FTIR spectrometer 20 measures thick film thicknesses. Spectrophotometer 10 and FTIR spectrometer 20 are typically not used to measure the thickness of the same layer within the same area of a sample.
Thus, a combined spectrophotometer and spectrometer unit that reduces sample handling costs, minimizes the horizontal area occupied by the resulting apparatus, and increases throughput as well as data correlation is needed.